The safety problem

Question 1

What is the definition of the safety problem?

Answer 1

Any computer system is safe with respect to the right r if the system cannot leak r.

Question 2

What is right leakage?

Answer 2

Right leakage is simply a phenomenon in which a given right r is added to an element in the ACM not already having the right r.

Let S1 and S2 be subjects and O be an object. S1 can read O whilst S2 has no rights over O. This is the original state of the ACM. A leakage would occur if a new right was given to any of the subjects, say S2 is given write permission. Then the system would leak the right w.

Question 3

Is right leakage always bad?

Answer 3

No, leakage is not always bad. Whenever a subject has a legitimate need for a right it does not hold, granting that right is will constitute leakage even if it is of no harm to the system.

Question 4

What is the fundamental safety question?

Answer 4

Is there a generic algorithm that allows us to determine whether a given protection state Q0 is safe with respect to a given right r.

The answer to the question is no, unless the system is mono-operational.